People cannot control their emotions. It only takes a few seconds to hear news about this and that terror attack, and this and that rape gang and go on this site and vent frustration on some random muslims.
The emotion isn't pure anger. It is a tribal emotion, of "their tribe attacked our tribe and we will attack theirs back". A very ancient animalistic urge.
Muslims get it too, presumably after they hear of this and that bombing of a hospital in Iraq by an airstrike, and so on. They then come and make something anti-Western online.
I'm not brushing off all controversial posts as emotional urges, which would be unfair. There are many valid criticisms of the Quran, of muslims, fears of refugees, Western policy etc., but I forgive people who aren't acting civil because they just could not handle themselves.